Telephone call-indicating facilities



Sept. 30, 1947.

, E..M. BOARDMAN I TELEPHONE CALL-INDICATING FACILITIES I 2 Sheets-Sheet1 MACH/NE File d Dec. 1, 1944 MACH/NE RING/N6 I MACH/NE RING/N6 MACH/NERING/NC T0 LOCAL POWER MACH/IVE RING/MG sxrlwslow .nsr

NORMAL SE T MA CHINE RING/N6 lNVE/VTOR BY E M BOARD/VAN aamAg A TTORNL vPatented Sept. 30, 1947 U NIT ED STATES PAT 01 GB 2,428,007 TELEPHONE;CALL-IN DICAT-IN G FACHiI-"HES;

Edward M; Boardinan, Mountain Lakes; N; J1, as

signer to Bell" Telephone Laboratories; In'co'r porated; New York, N.Y'., ac'orporation-of' New York Application December 1, 1944, Serial No.566,053

9 Claims.

1 This invention relates to signaling facilities for use at telephonesubscribers stations and particularly to call-indicating facilitieswhich. function' to indicate at a subscribers station or at an extensionset associated therewith, the arrival. of a telephone call by timespaced signals of distinctive characters.

The signaling equipment'commonly used at tel ephonesubscribers stationscomprises an electromagnetic device; generally characterized as. a

ringer, which respondsto each application of ringing current to thesubscriber's line to produce sound signals of uniform intensity. Whilesuch signals function satisfactorilv t has been found desirable in someinstance' tt'o furnish a.

having a relatively low sound output; Such a" solution of theproblem,however, introduces the possibility that the tone signalswould be ofsuchasubdue'd character as to. be useles in arresting'theattention of thesubscriber when'loc'atedat any appreciable distance from the telephonewith the inevitable result that numerous calls incoming to'the'statio'nwould go unanswered and wouldbe lost.

It is. the object of this invention to provide eall indicating equipmentfor use with a telephone subscrihers'telephone set or as an. extensionset associated therewith, which responds. to the'application' ofsignaling current to thesu'bscribers line so as to indicate the'arrivalof a call first'by a signal, at the'telephone set, oflo-Wattention-arresting quality, and subsequently, either atithe telephoneset or at an extension set by'a signal of higherattention-arrestingquality.

This object is attained in accordance with a feature of the invention bythe provision of signaling facilities which render it possible for asingle audible signalingdevice to operate and emit sound signals ofdifferent characters at time spacedintervals at. a telephone subscribersstation, or to supplement the regular audible signal atla subscribersstation with either visual or audible signals at ail-extension set, thelatter signals having a higher attention-arresting quality than theregular signal associated with the subscriber telephone set.

The signaling-facilities contemplated by-this invention are desirablenot only at subscribers premises whichare equipped with asingle-telephone, but also find particular utility:- in many 2locationsat wh'icli there are'located-a pluralityof independent"telephones; in that the" supplementary signal of higher"attention-arresting quality affords a; Iiibslt'il/(i identification 0fthe" line" on which a call'has arrived. m tlie'Iatterca'se-thesupplementary signaling equipment may" be rurnished as" an extension setand" located either in close proxiinitwwithtlie" telephone setor at adistance therefrom so that "positive identification of' the called linemay; be given the" subscriber when he isiloc'ated at a point remotefrom" his telephone. I

Theinvention. will be readily uriders'tood from the" following detaileddescription madewitlt reference" to the accompanying drawings in* which:

Figs. 11,2 13; 4, 5and6*illi1sti'ate"diagrammatically severalembodiments of the invention which provide 'twouistincti typesofealrsignels' at timesp'ace'd intervals;

Fig. '7. is a circuit diagram showing a number of signaling facilities?arranged" for" mounting in an" extension set, and" indicates a;plurality; of possible connections which may be effected to producediiferent signaling)- arrang'ements';'

Fig. 8*is' a schematic"representation"ofa slow-- to op'erate switchingdevice which is particularly suitable for use in the circuits shown inFigs. 1, 4-, 5an'cl 6; v

Fig; 9 shows" the switch of Fig. '7" fully" operated; and v Fig. 10illustrates a; ringer. design which is particularlysuitable forus'e-asthe audible" signal in'the 'circuit'shown' in Fig, 3

It 'is' to 'be understoodthat the signaling" facilities' illustrated'inFigs: 1, 2; 3; 4, 5- andfdmay be used not only as" replacements for theregular ringer equipment located at a subscribers. station, but alsomaybehousediin'a'separate'container as an extension set' and" located ata point remote from thesubscribers' telephone, if conditions warrant it.

In Fig. lthereis shown at llla'sourcje'of'machine ringing currentwhichis generally l'ocated at'a, central office. The telephoneline extendingfrom the central'offioe' to a" subscribers"station comprises the tipand'ring'conductors H and" [2, respectively. The equipment shown'totheright of the source l-Ui ashereinbeforeindicatedmay replace theregularringer equipment at the subscribers 'stationcr may be housed inaseparate container and located elsewhere' on the premises asan'extension' set. For descriptive purposes-only, itwill be assumed-thatthis;ec1uipn'ient replaces the usual ringer 'equipmen-ta. As. thestation a ringer tarot well-known design is: connected =acro'ss" twobranch circuits each of which is bridged across the tip and ringconductors l l and I2 of the subscribers line. One circuit includes adry rectifier M, a resistance l and a condenser H5. The other circuitincludes a dry rectifier I! and a condenser IB. One ringer terminal isconnected to a point between the resistance l5 and the condenser-I6,while the other terminal of the ringer is connected to a point betweenthe rectifier IT and the condenser IS. The rectifiers l4 and I! arepoled oppositely. A slow-to-operate switch is is connected across thetip and ring conductors l l and ii of the subscribers line in serieswith a condenser 20 and a current limiting resistance 2 l. The armatureand contact of the switch or relay [9 are so connected in the circuitthat they effectively short-circuit the rectifier I! when the relay isfully operated.

The rectifiers l4 and l! and the condensers l6 and I8 are sointerconnected with respect to the ringer l3 that they function as avoltage doubling arrangement. In accordance with the theory of operationof the voltage doubling arrangement, the condensers l6 and I8 arealternately charged from the alternating current from source Ill to thesame polarity through the rectifiers H and. I4, so that voltagesapproximately equal to the peak value of the alternating current sourceare maintained across each condenser. Since the condensers I6 and I8 arein series with respect to the ringer i3, the ringer is subjected to anoperating direct current voltage substantially equal to twice thevoltage of the alternating current source and this voltage is maintainedas long as the alternating current is applied. Since, by design, theringer is the well-known polarized type of device, it operates upon theapplication of the machine ringing source to the line to cause itsclapper to strike one of its associated gongs. When the signalingcurrent is interrupted, the ringer is deenergized and the biasing springof the ringer restores the clapper to its normal position against theother gong causing the said other gong to be actuated. Thus, if theringing current is connected to the line for two (2) seconds anddisconnected therefrom for four (4) seconds in accordance with usualpractice, each gong of the ringer will be struck once every six (6)seconds. This operation of the ringer produces an audible signal of lowattention-arresting quality.

Before proceeding further with the description of the operation of thecircuit shown in Fig. 1, it may be well to refer to Figs. 8 and 9 whichillustrate a preferred form of slow-to-operate switch which may be usedas the slow-to-operate switch conventionally illustrated at IS in Fig.1.

The switch shown in Figs. 8 and 9 functions to effect a circuit closurein response to a predetermined number of spaced impulses impressed onits winding.

The structure of this switch includes a sealed vertical tube 80 having apool 8| of mercury in the bottom and a hollow cylindrical armature 82open at the top and closed at the bottom except for a small orifice 83.In its normal position, as shown in Fig. 8 the bottom of the armature isforced down against the surface of the mercury pool 8| by mean of alimiting stop 84 so that a small portion of the mercury rests within thearmature 82 and in physical communication, through the small opening 83in the bottom, with the remainder of the mercury mass in the tube below.One terminal 85 of the switch projects down from the top of the tubeinto the interior of the armature while the other switch terminal 86projects up from the bottom of the tube into the mercury mass 8|. Whenthe operating coil 81 of the switch is energized for an interval by apulse of current from the machine ringing source the armature 82 isforced down against the mercury pool 8|, causing a corresponding amountof the mercury to flow up through the orifice 83 into the armature. Onthe next open period of the coil, the forces of surface tension in themercury cause it to flow back through the orifice, moving the armatureupward toward its normal position. By choosing the length of the currentimpulses and the length of the spacing between them, the level of themercury within the armature may be made to rise and fall successively,each rise exceeding somewhat the succeeding fall, thus causing thesurface of the mercury within the armature to approach stationaryterminal 85. After the desired number of impulses have been received,the surface of the mercury engages the end of the contact 85 and clingsthereto by adhesion. On the succeeding fall of the mercury level, theforce of adhesion causes the mercury body to become displaced so as notto break the contact that has been established. On the next fewsucceeding interruptions in the operating current the level within thearmature may fall somewhat below the stationary terminal, but theadhesion of the mercury to the terminal serves to maintain constantengagement until the general level of the mercury rises above the end ofthe terminal and the armature reaches its full downward position. Thusthe circuit once established is safeguarded against opening. Obviously,when the energizing source is disconnected for any appreciable time thearmature restores to its normal position and the circuit is opened.

Reverting back to Fig. 1, the slow-to-operate switch just describedresponds in a step-by-step manner to the successive applications ofsignaling current to the line and after a predetermined number of suchapplications, the switch contacts are closed to effectivelyshort-circuit the rectifier I1. Thus, the ringer is subjected toperiodic impulses of one polarity which traverse the rectifier l4,resistance l5, ringer l3 and condenser 18, whereupon each gong of theringer is struck once for each half-cycle of current passed by therectifier l4. One gong is struck upon the application of each half-cycleof current to the line and the other gong is struck between successivehalf -cycles of the same polarity by virtue of the clapper restoringaction of the ringer biasing spring.

Thus the ringer I3 is operated to emit a preliminary audible signal oflow attention-arresting quality and, after a predetermined interval oftime, that is, when relay I9 operates, it is operated to emit asecondary audible signal of greater intensity. When the subscriberresponds to the signal, the ringing current is tripped in the wellknownmanner whereupon the ringer I3 is silenced and the switch l9 restores tonormal.

The condenser 20 in series with the winding of the switch [9 is used fordirect current blocking purposes. The resistance I 5 is provided toreduce the over-all shunting eflect of the condenser 16 when the switchI!) operates to short-circuit the rectifier H.

For purposes of simplicity, the telephone sets at the subscribersstations of Figs. 1, 2, 3, 4, 5 and 6 have not been illustrated.

The circuit of Fig. 2 is substantially identical to that of Fig. 1,except that the slow-to-operate relay '29 is provided with a transfercontact arrangement so that when the relay operates, the circuit torectifier 24 is opened, the rectifier 21 is shortcircuited and theringer 23 is connected directly across the line by way of the frontcontact and armature of relay 29.

The preliminary signal emitted by the ringer 23 is identical to thepreliminary signal emitted by ringer !3 (Fig. 1), but the secondarysignal is produced by both half cycles of applied current since bothrectifiers 24 and 21 are rendered ineffective by the operation of switch29.

While the switch illustrated in Figs. 8 and 9 is not shown equipped witha transfer contact and therefore would not function in Fig. 2, it is tobe understood that any slow-to-operate switch having armature controlledfront and back contacts will serve in its place. The particular designof slow-to-operate switch does not, of itself, constitute a part ofthepresent invention and the design illustrated in Figs. 8 and 9 hasbeen described merely as an example of one type of switch particularlysuitable for use in the circuits of Figs. 1, 4, 5 and 6.

Fig, 3 illustrates an arrangement whereby the preliminary ring is at atwenty-cycle rate on only one gong of the ringer, twenty cycles beingthe frequency of the alternating current source included in the machineringing supply in. For

'this purpose a ringer of the design illustrated in Fig. may be used.This ringer is similar in design to the ringer illustrated in UnitedStates Patent 2,269,108 to J. D. Hubbell and C. F. Wiebusch and diiferstherefrom in that the leaf spring 9!] is placed on the stop side of thearmature air-gap; in other words the spring 90 is positioned on theringer structure to oppose the normal bias spring 9| and to leave asmall space at 92 available on the stop side.

A gas-filled tube 3| is connected in series with the ringer33 by way ofa current limiting resistance 32 and is poled to pass pulses through theringer in such a direction as to pull the armature up on the stop side,that is, so as to cause the clapper to strike gong 93 (Fig. 10). Thisproduces a low volume ring on this gong as a preliminary signal of lowattention-arresting quality. The relay, or switch 34, functions after apredetermined interval of time to effectively short-circuit therectifying tube 3| and to connect the ringer 33 directly to the line byway of the condenser 35. The ringer then functions in a manner such thatboth gongs are struck by the ringer clapper. The tube 3|, when activatedalso acts as a preliminary visual signal.

The resistance 36 and condenser 31 in circuit with the winding of switch34, serve the same purposes as do resistance 2| and condenser 20,respectively, of Fig. 1.

While Fig. 4 is substantially identical to Fig. 3, it is intended toillustrate the use of a glow lamp 4| which is non-rectifying in itsoperating function, whereas the tube 3| of Fig. 3 functions as arectifier to control the sound signal emitted by the ringer 33. In Fig.4 the tube 4| is connected in series with ringer 43 and functions as avisual signal when the ringing source In is connected to the subscriberline. After a predetermined time interval the delay switch 44 operatesto short-circuit the visual signal 4| and to connect the ringer 43directly to the line whereupon the ringer operates in a normal manner togive forth a secondary audible signal. It will be noted that the currentwhich traverses the visual signal 4| alsopasses through the ringer sothat the ringer may function as a preliminary audible signal if themagnitude of the current is sufiicient to cause the operation of theringer clapper. However, the ringer under this condition will operateonly in a subdued manner. By adjusting the value of the tube current theoperation of the ringer as a preliminary audible signal may beprevented, if desired. 7

Fig. 5 distinguishes from Fig. 4 essentially only in the substitution ofa resistor 54, having a negative temperature coefiicient of resistance,for the slow-to-operate switch 44 of Fig. 4. The resistor 55 isconnected in parallel with the lamp 5| and in series with the ringer 53.The characteristics of this resistor are such that when it is cold itsresistance is so high as to prevent sufiicient current to pass throughthe ringer 53, but with the passage of time, its resistance drops to anegligible value and the ringer operates at full intensity. When theresistance of resistor 54 reduces to a negligible value the lamp 5| iseffectively short-circuited. The same types of signals are obtained fromthe circuit arrangement of Fig. 5 as are obtained from th circuitarrangement of Fig. 4.

In some cases it may be desirable to provide a preliminary visual signalof greater intensity than can be obtained with the visual signal devicesmentioned hereinbefore. The circuit of Fig. 6 provides such a service.In this showing the subscribers line is bridged by two ringing currentresponsive circuits each including a condenser '|0| or I92, aslow-to-operate relay M3 or 'IM, and a current limiting resistance I05or I06. Relay )3, when operated, connects the ringer it] in series withcondenser Hi3 across the line. Relay Hi4, when operated, connects thelamp signal N39 to a local source of power. The relays H13 and W4 may beof the type shown in Figs. 8 and 9 and adjusted so as to have differenttime operate characteristics. With the relay H14 adjusted to operatequicker than relay m3 it is obvious that upon the application of ringingcurrent to the line for a brief interval of time the lamp I09 will belighted as a visual signal of relatively high intensity. Subsequently,relay I03 operates .to connect the ringer to the line causing it to givea secondary audible signal. When the call indicated by the operatedsignals is answered the ringing current is tripped in the well-knownmanner and the signals are rendered inoperative.

The signaling arrangement shown in Fig, 6 may be modified in accordancewith the requirements of th subscriber to provide, for example, theusual ringer signal followed by a visual signal.

'In this case the extension set would supplement the normal set andwould include the condenser Iii-l, slow-to-operate relay I33, currentlimiting resistance I05, all connected across the line, a lamp signalI09 and a power circuit control relay. The normal ringer would respondto the incoming signaling current to give th usual audible signal andthe slow-to-operate relay me would, after a predetermined interval oftime, operate to connect the power circuit control relay to the line soas to subject it to the incoming signaling current. This relay respondsto the incomin ignaling current to complete a local power circuit whichincludes the lamp signal [09. Thus, if the normal ringer signal is notanswered within the predetermined interval of time, the lamp signalwould be actuated to provide a secondary visual signal. Such a lampsignal could be located at some distance from the telephone with whichthe normal ringer is associated, if desirable;

Obviously, the signaling arrangement of Fig. 6,

agaspov as well as those of the preceding figures maybe used assubstitutes for the regular signaling device at a telephone station ormay be housed in separate containers as extension sets for locationelsewhere on the subscribers premises and to supplement the regularringer at the subscribers station. In the latter case, the regularringer would respond immediately to the ringing current applied to theline and if the call is unanswered, the ringer signal would be followedby the operation of the visual signal 109 and of ringer 101 in thatorder at the locality of the extension set. It is to be understood thatthe extension sets may be located anywhere in the subscribers premisesand need not be closely associated with the subscribers telephone.

The arrangements thus fardescribed illustrate various means forobtaining time-spaced telephone call signals of diiferentattention-arresting qualities. It is apparent that the various signalsmay be combined in manners other than those described. It is alsoapparent that the facilities described are particularly suitable forinstallation as extension sets and also as substitutes for the regularsignaling equipment located at a subscribers station.

In Fig. 7 several of the ringing facilities previously described areillustrated as housed in a separate housing constituting an extensionset with switching means for connecting the various elements to thesubscribers line in different combinations. The arrangement illustratedserves primarily as a means for conveniently demonstrating to aprospective subscriber several different types of signals which areavailable to him and which embody the idea of providing a preliminarysignal of low attention-arresting quality followed by a signal ofgreater intensity. The manual switching devices are represented bybroken lines terminated by an arrow-head.

To the left of the broken line box shown in Fig. 7 there is indicatedthe regular machine ringing source 69 which is located at a centraloffice and which, in a well-known manner, is connected to the lineconductors El and 62 extending to a subscribers station. The manuallyoperable switching devices are designated 1, 2, 3', 4, 5, 6, l and 8. Ifit is desirable that the subscribers station be furnished with theregular audible type of signal only, the switch 8' would be closed tothe point 8 on the line conductor 61 and all other switches illustratedwould be opened. Under this condition the ringer 53 would respond to theapplication of signaling current from the source 51! to the subscribersline {ii-52, inthe well-known manner, and its operation would cease whenthe ringing current was tripped in the usual manner when the subscriberanswered the call.

Should it be desired that a visual signal be provided which responds tothe incoming ring at the same time that the ringer 63 operates, then theswitch 6 would be closed to the point 5 on the line conductor 6! toeffectively connect the gas tube 5 2 in parallel with the ringer 63across the line. Obviously, both the ringer 63 and the tube 64 respondsimultaneously to the signaling current from the source 633 to provideboth audible and visual indications of an incoming call.

To provide a subdued audible signal only, switch 8 is operated tothe-point 2, switch 3 to the point 3 and switch 4 to the point 5, andall other switches are opened. Under these conditions the rectifiers 65and 58, condensers 61 and 6B and the ringer 59 are connected across the.

line 61-62, as a voltage doubling arrangement similar to that shown inFig. 1, except that the slow-to-operate switch '19 is not connected toalter the signal after the elapse of a predetermined interval of time.This circuit arrangement results in the operation of the ringer 69 in amanner such that each ringer gong is struck once every six seconds,assuming the ringing current to be connected to the line for twoseconds, and disconnected therefrom for four seconds, as described inconnection with Fig. 1. V

The subdued signal described in the preceding paragraph may be followedby a sound signal of greater attention-arresting quality by operatingswitch 7' to the point I and switch 2 to the point 2. Thus with switchesl, 2', 3', 4 and I operated, the circuit arrangement shown in Fig. 1 iseffected, and as described in connection with Fig. 1, the ringerfunctions first, to emit a sound signal of low attention-arrestingquality as a preliminary signal and then, after a predetermined timeinterval, it functions to give forth a signal of greater intensity.

With switch 6, switch I, and switch 2 closed to their respective contactpoints 6, l and 2 and all other switches open, the tube 64 wouldfunction as a preliminary visual signal and after a predetermined timeinterval, switch 10 would function to connect the ringer t9 andcondenser 68 across the line to cause the ringer to emit an audiblesignal.

By operating switch 8' in place of switch 6 in the previous paragraph,the ringer 63, which may be adjusted to produce a signal of reducedsound intensity would operate as a preliminary audible signal of lowattention-arresting quality and after a predetermined time interval theswitch 10 would function to connect the ringer B9 in circuit.

The ringer 63 is illustrated in Fig. 7 as being connectable to the linethrough the medium of a switch 8"merely for purposes of simplificationof the description of the various combinations of signals obtainable.Actually this ringer would be permanently connected to the line at asubscribers station in the well-known manner,

' and in demonstrating the possibilities of the extension set and of thesignal combinations obtainable thereby the normal connection of theringer at the telephone set would be opened, or closed as desired.

What is claimed is:

1. The combination, in a signaling system, of

a line, a source of current, a signal device, means connecting saidsignal device to said line by way of a branch circuit having electriccharacteristics such that said signal device is caused to operate in aparticular manner as a preliminary signal when said source of current.is connected to said line, and electromagnetic time delay meansresponsive to current from said source for altering the electriccharacteristics of said connecting means, whereby said signal device iscaused to operate in a different manner by current from said source, asa secondary signal,

2. In a telephone signaling system, a line circuit, a station thereon, asignal device at said station, a source of signaling current, a circuitnormally connecting said signaling device to said lineand in which saidsignaling device operates in a particular attention arresting mannerupon the application of current from said source to said line, andelectromagnetic switching means responsive to the application of currentfrom said source to said line and efiective a predetermined interval oftime after the said application for altering the electriccharacteristics of said normally connected circuit, whereby saidsignaling device is operated in a different attention arresting mannerby current from said source.

3. In combination, a station, an audible signal device at said station,a line on which said station is located, a source of Signaling current,a current rectifying device, a circuit bridged across said lineincluding said signal device and said current rectifying device, andtime delay means responsive to current from said source when applied tosaid line, for efiectively excluding said current rectifying device fromsaid bridged circuit.

4. In combination, a station, an audible signal device at said station,a line on which said station is located, a source of signaling current,a circuit connecting said signal device to said line comprising a pairof parallel arms connected across said line, each arm including ahalf-wave rectifier and a condenser and each having a point intermediateits respective rectifier and condenser to which opposite terminals ofsaid signal device are connected, a time delay device responsive tocurrent from said source when applied to said line, and means controlled:by said time delay device for efiectively connecting said signal deviceto said line independent of said rectifiers.

5. In combination, a line, an audible signal having an operatingwinding, a source of signaling current, means connecting said audiblesignal to said line comprising a voltage doubler of the rectifier typeconsisting of two series circuits each including a unidirectionalcurrent conducting device and a condenser connected across said linewith the winding of said signal device connected between said circuitsto a point in each circuit between its respective unidirectional currentconducting device and condenser, and means responsive to the applicationof current from said source to said line for a predetermined interval oftime for short-circuiting one of said unidirectional current conductingdevices.

6. The combination, in a signaling system, of a line, a ringercomprising a pair of gongs, a striker, a restoring spring for saidstriker and an operating winding, a source of alternating signalingcurrent adapted to be connected to said line at repeated time intervals,circuit means connecting the operating winding of said ringer to saidline and including rectifying means which function to cause theoperating winding of said ringer to be energized once for eachapplication of current from said source to said line, whereby saidstriker actuates one of said gongs for each application of current tosaid line and actuates the other gong, under the restoring action ofsaid spring, for each disconnection of said source of current from saidline, and means operating in response to a predetermined number ofapplications of current from said source to said line to disable therectifying means of said circuit connecting means whereby said strikerfunctions to repeatedly actuate both said gongs during each applicationof current from said source to said line.

7. The combination, in a telephone signaling system, of a telephoneline, an electromagnetically operated ringer connected to said line, asource of signaling current, rectifier means connected in circuit withsaid ringer for causing said ringer to operate in a particular attentionarresting manner in response to the application of current from saidsource to said line, and other means responsive to current from saidsource for altering the connection of said rectifier means to saidringer whereby said ringer is caused to operate in a difierent attentionarresting manner in response to an application of current from saidsource to said line of a predetermined time duration.

8. In combination, a line, a source of machine ringing current connectedto said line for intermittently applying signaling current therefrom tosaid line, a signal on said line, circuit means connecting said signalto said line and having. characteristics such that said signal operatesin a characteristic attention arresting manner on each application ofsignaling current to said line, and electromagnetic switching meansresponsive to a predetermined number of applications of current fromsaid source to said line for altering the characteristics of saidcircuit connecting means whereby said signal operates in a differentcharacteristic attention arresting manner on applications of currentfrom said source to said line in excess of the said predeterminednumber.

9. The combination, in a signaling system, of a line, an audible signaldevice associated with said line, a source of signaling current, acircuit connecting said audible signal device to said line and includingan element normally electrically in circuit with said audible signal device to cause said signal device to operate in a distinctive manner as apreliminary signal upon the application of current from said source tosaid line, a slow operating electromagnetic switching device connectedto said line so as to be subject to current from said source, and meanscontrolled by said switching device when operated for abruptly alteringthe normal connection of said element with said signal device wherebysaid signal device is caused to operate in a different distinctivemanner as a secondary signal.

EDWARD M. BOARDMAN.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 2,332,448 Inglis Oct. 19, 19432,151,726 Allen Mar. 28, 1938 2,180,334 Bredehoft Nov. 21, 19382,291,615 Ferrell Aug. 4, 1942 2,302,544 Halligan Nov. 17, 1942

